The human cranium was found during World War II mining operations in 1942, in a cave littered with Ice Age cave bear remains.

Recently the fossil was radiocarbon dated to 33,000 years ago and thoroughly examined, revealing the controversial anatomical feature.

The otherwise human skull has a groove at the base of the back of the skull, just above the neck muscle, that is ubiquitous in Neandertal specimens but has never been seen in the remains of a modern human, argues study leader Erik Trinkaus, an anthropologist at Washington University in St. Louis, Missouri.

"I was frankly quite surprised to see it when I was looking at the specimen," Trinkaus said. "My first reaction was, that shouldn't be there."

Writing in the August issue of Current Anthropology, Trinkaus and his colleagues say that the skull supports interpretations of other remains found in France, Romania, and the Czech Republic that also have "archaic" or unusual features suggesting interbreeding.

Ancient Debate

Archaeological evidence shows that humans and Neandertals (often spelled Neanderthals) both lived in Europe for several thousand years after the arrival of modern humans about 35,000 years ago. (Learn more about human migrations.)

But the relationship between the two species is hotly debated.

Many researchers believe that modern humans killed off or simply outcompeted Neandertals until the latter went extinct.